Typical non-magnetic radiators or scatterers obey reciprocity and will exhibit a time symmetric response. For example, radiation patterns of an antenna in transmit and receive modes will be identical. This presents challenges in complex environments in which a directive antenna is typically forced to listen to its reflected echo. In the context of energy harvesting, solar panels and thermophotovoltaic cells are tailored to be highly absorbing in the spectral range of interest, typically in the visible or infrared range. However, reciprocity and time-reversal symmetry fundamentally requires these highly absorbing structures to also be very good emitters in the same spectral range. This fundamental relationship implies that, as the panels heat up, they are required to emit a significant portion of absorbed energy in the form of thermal infrared emission towards the source, causing a reduction in efficiency. In another example, an incident wave upon a metasurface is scattered with some efficiency towards some direction, then a backward propagating wave from that direction will be equally coupled to a backward propagating wave towards the direction of original incidence thereby causing a reduction in efficiency.
Over the years, a few groups have pointed out that, by preventing reciprocity, one may be able to overcome these challenges. Reciprocity can be prevented by using magnetic materials, such as ferrites. However, such materials are bulky and made of expensive rare earth materials and require large magnetic field biasing. Alternatively, reciprocity can be prevented using non-linear materials. However, the use of non-linear materials results in undesirable signal distortion and a power dependent response.
As a result, there is not currently an effective means for eliminating the reciprocity constraints in radiating and scattering systems, such as antennas, metasurfaces or frequency selective surfaces.